Magnetron oscillators

ABSTRACT

A magnetron oscillator having the normally provided cavities of a magnetron oscillator, has a further resonant cavity coupled to one of the normally provided cavities by an inductive loop. The further cavity is provided with two electrodes facing one another, the opposing faces being coated with a secondary electron emissive material. The two electrodes are spaced apart such that an electron released from one surface takes a time equal to half a cycle of the R.F. voltage developed between the two electrodes to cross the gap to the other electrode thereby producing a multipactor discharge between the two electrodes. While multipactor discharging occurs the further cavity is effectively short circuited so that the magnetron frequency of oscillation is the resonant frequency of its anode cathode resonator. The multipactor discharge is stopped by application of a D.C. voltage to the two electrodes so that the further cavity pulls the magnetron frequency.

United States Patent Pickering July 24, 1973 MAGNETRON OSCILLATORS Primary Examiner-John Kominski [75] Inventor: Alan Hugh Pickering, Springfield, Amey Dnald wght et England ABSTRACT [7 3] Assrgnee: English Electric Valve Company Limited, London England A magnetron oscillator having the normally provided cavities of a magnetron oscillator, has a further reso- [22] Filed: May 4, 1972 nant cavity coupled to one of the normally provided 1 J cavities by an inductive loop. The further cavityis pro- [2 1 Appl No 250358 vided with two electrodes facing one another, the opposing faces being coated with a secondary electron [30] Foreign Application Priority Data emissive material. The two electrodes are spaced apart May 4, 1971 Great B i in 13,036/71 such that an electron released from one surface takes a time equal to half a cycle of the R.F. voltage devel- [52] US. Cl 331/90, 315/3957, 331/89, p betwe n the two electrodes to cross the gap to the 331/90, 332/5, 333/13 other electrode thereby producing a multipactor dis- [51] Int. Cl. 1103!: 9/10 harg we n he two el ctrodes. While multipactor [58] Field of Search 331/90, 89, 87; i charging occur the further cavity is effectively short 315/3957; 333/13; 332/7, 5, 25 circuited so that the magnetron frequency of-oscillation is the resonant frequency of its anode cathode resona- [56] References Cited tor. The multipactor discharge is stopped by applica- UNTED STATES PATENTS tion of a D.C. voltage [0 the {W0 electrodes SO that the 2,674,694 4 1'954 Baker sis 39.57 further pulls the magnum" frequelcy' 8 Claims, 1 Drawing Figure 17 I \l J 22 2 I U 21 7 19 i B MAGNETRON OSCILLATORS This invention relates to magnetron oscillators.

In certain systems such as radar systems there is a requirement for a magnetron oscillator having an oscillation frequency that can be rapidly changed from one frequency to another frequency.

The present invention seeks to provide such a magnetron oscillator.

According to this invention a magnetron oscillator comprises a cathode and anode structure having a plurality of cavities therein; a separate resonant cavity coupled to one of the cavities in the cathode and anode structure; and a pair of spaced electrodes the surfaces facing one another of which are formed of secondary electron emissive material and the positioning and dimensioning of which electrodes are such that if a suitable voltage is set up between the electrodes a mu]- tipactor discharge will occur therebetween.

A multipactor discharge occurrs between a pair of electrodes with secondary electron emissive surfaces if an r.f. voltage is set up therebetween and if an electron leaves one electrode and impacts on the other electrode after a half cycle of the r.f. causing the release of several electrons from the electrode which in turn travel to and impact on the said one electrode after a further half cycle of the r.f. and has the same effect as the first electron. The number of electrons leaving the electrodes rapidly builds up until a saturated discharge is reached in the gap between the electrodes. To turn off the discharge or to prevent the discharge a suitably large d.c. voltage can be applied between the electrodes or alternatively a large transverse magnetic field may be applied. By the expression secondary electron emissive material is meant a material with the secondary electron emission ratio greater than unity.

In operation of a magnetron oscillator in accordance with the invention the r.f. energy will couple into the resonant cavity from the cathode and anode structure cavity and this will result in an r.f. voltage being set up between the electrodes. Preferably the electrode arrangement is made such that this r.f. voltage corresponds to said suitable voltage so that, in the absence of an inhibiting large d.c. voltage or transverse magnetic field, a multipactor discharge will occur.

In the preferred embodiment connection means are provided for applying a voltage to said electrodes whereby a d.c. voltage may be applied during operation to effect inhibition or switching off of a multipactor discharge set up between the electrodes.

Preferably said separate cavity is formed on the outer wall of the cathode and anode structure of the magnetron oscillator.

Preferably said cavity is coupled to one of the cavities in the cathode anode structure by a coupling loop.

Preferably one of said electrodes is provided with a heater.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a part-sectional diagrammatic view of a magnetron oscillator in accordance with the invention.

The magnetron oscillator shown in the drawing has a cathode l and a hollow cylindrical anode 2 with vanes 3 that, extend inward radially towards the cathode 1. Mounted on the anode 2 are magnetic pole pieces 4,5. The spaces between each pair of adjacent vanes 3 constitutes a resonant cavity, and the particular arrangement of vanes and resonant cavities that may be used may be as well known per se, e.g. as in a rising sun magnetron.

The cathode l of the magnetron oscillator is supported on an insulated vacuum tight support 6 which is connected to the pole piece 4. The anode 2 of the magnetron carries an output port 27, to which a waveguide may be connected. The output port 27 has a glass window 28 which forms a vacuum tight seal, and which is transparent to the output signal of the magnetron oscillator.

Formed on the outer wall 7 of the anode 2 there is a further resonant cavity 8 which is provided with two electrodes 9, 10. The electrode 9 is a hollow cylindrical electrode that has an end plate 11 made of a material that is thermionically emissive and secondary electron emissive. The electrode 9 makes electrical contact with the wall 12 of the resonant cavity 8. A heater 13 is mounted inside the electrode 9 and a current is supplied to the heater 13 along a lead 14 which passes into the electrode through an end cap 15 which is insulated from the rest of the electrode by an insulating ring 16. The electrode 10 is a cylindrical electrode and is provided with an end plate 17 made of secondary electron emissive material. The electrode 10 passes through an aperture 18 in the wall 19 of the resonant cavity 8 and is supported by an insulating vacuum tight support 20. Mounted inside the cavity 8 adjacent to the wall 19 there is an apertured plate 21 which is positioned so that the electrode 10 passes through the aperture. This plate carries a cylinder 22 which surrounds the electrode l0 and acts as an R.F. choke. The electrode 10 is provided with a connecting lead 23, and the cavity is also provided with a connecting lead 24, whereby a voltage may be applied between the electrodes 9 and 10.

The resonant cavity 8 is inductively coupled to one of the cavities of the magnetron oscillator by an inductive coupling loop 25 which passes through an aperture 26 in the wall 7 of the anode 2. The loop 25 is generally of an S shape, but one of the loops forming the S is in a plane at to the plane containing the other loop forming the S.

When the magnetron is operated the cavity is strongly excited by energy coupled from the cavity in the anode of the magnetron by the coupling loop 25 and, in the absence of an electron discharge, some tens of kilovolts of R.F. are developed between the electrodes 9, l0 and the frequency of oscillation of the magnetron is pulled by the cavity 8 to a predetermined frequency.

If the magnitude and frequency of the R.F.- voltage developed between the electrodes are related to the separation between the electrodes 9 and 10 so that an electron released from one surface takes a time equal to half a cycle to cross the gap a multipactor discharge will occur between the electrodes. To assist in the initiation of the discharge in the illustrated embodiment a heating current is fed through the heating element 13 via the leads l4 and 24.

A multipactor discharge occurs if an electron leaves one electrode at an appropriate time in the cycle and impacts on the other electrode a half cycle in time later, causing the release of several electrons from that electrode which in turn travel in the opposite direction and each have the same effect on the said one electrode. The number of electrons released rapidly builds up until a saturated discharge is reached in the gap between the electrodes. While the multipactor discharge is occurring the voltage across the electrodes 9, it) falls to about 259-560 volts and the cavity 8 is effectively short circuited, and so the frequency of oscillation of the magnetron returns nearly to the resonant frequency of its anode cathode resonator.

The multipactor discharge can be stopped by applying a suitable D.C. voltage to the electrodes 9 and 110, the overall effect being that the magnetron frequency of oscillation may be switched between two different values by the absence or application of a suitable bias voltage to the electrodes via leads 23 and 24. Alternatively transverse switching off of the multipactor discharge may be effected by applying a suitable magnetic field to the gap between the electrodes. When the discharge is stopped the frequency of oscillation of the oscillator is again pulled by the cavity 8 to the predetermined frequency.

Although the invention has been described with reference to a magnetron having one further resonant cavity 8 it is possible to produce magnetrons having a plurality of such additional cavities.

I claim:

1. A magnetron oscillator selectively tunable to a first and a second oscillating frequency comprising a cathode and anode structure having a plurality of cavities therein; a separate resonant cavity coupled to one of said cavities in the cathode and anode structure so as to determine the oscillating frequency of said magnetron oscillator; a pair of spaced electrodes within said separate cavity each electrode having a surface facing the other electrode, each surface formed of secondary electron emissive material, and the spacing between the two electrodes being suitably dimensioned to enable a multipactor discharge to be initiated and sustained between said electrodes so as to cause the oscillator to oscillate at said first frequency; and means for extinguishing said multipactor discharge whereby the oscillator oscillates at said second frequency.

2. A magnetron oscillator according to claim 1 wherein said means for extinguishing comprises means for applying a voltage to said electrodes whereby a d.c. voltage may be applied during operation to effect inhibition or switching off of a multipactor discharge set up between the electrodes.

3. A magnetron oscillator according to claim 2 wherein said separate cavity is formed on the outer wall of the cathode and anode structure of the magnetron oscillator.

4. A magnetron oscillator according to claim 3 wherein said separate cavity is coupled to one of the cavities in the cathode anode structure by a coupling loop.

5. A magnetron oscillator according to claim 4 wherein one of said electrodes is provided with a heater.

6. A magnetron oscillator selectively tunable to a first oscillating frequency and to a second oscillating frequency, said oscillator comprising in combination:

magnetron anode-cathode resonator means for nor mally resonating at a frequency near to said first oscillating frequency, said resonator means including a magnetron resonant cavity; a separate resonant cavity capable of causing said resonator means to oscillate at said second oscillating frequency;

coupling means coupling said magnetron resonant cavity to said separate resonant cavity for causing said oscillator preferentially to oscillate at said second selected frequency;

multipactor discharge means in said separate resonant cavity for effectively short circuiting said separate resonant cavity whereby said oscillator oscillates at said first oscillating frequency; and

means for selectively controlling said multipactor discharge means to select the frequency at which said oscillator oscillates.

7. A magnetron oscillator as defined in claim 6 wherein said multipactor discharge means comprises spaced electrodes between which multipactor discharge tends to occur at said second oscillating frequency due to the magnitude of energy coupled to said separate resonant cavity by said coupling means.

8. A magnetron oscillator as defined in claim 7 wherein said means for selectively controlling said multipactor discharge means comprises means for selectively applying a dc voltage to said electrodes to extinguish multipactor discharge therebetween. 

1. A magnetron oscillator selectively tunable to a first and a second oscillating frequency comprising a cathode and anode structure having a plurality of cavities therein; a separate resonant cavity coupled to one of said cavities in the cathode and anode structure so as to determine the oscillating frequency of said magnetron oscillator; a pair of spaced electrodes within said separate cavity each electrode having a surface facing the other electrode, each surface formed of secondary electron emissive material, and the spacing between the two electrodes being suitably dimensioned to enable a multipactor discharge to be initiated and sustained between said electrodes so as to cause the oscillator to oscillate at said first frequency; and means for extinguishing said multipactor discharge whereby the oscillator oscillates at said second frequency.
 2. A magnetron oscillator according to claim 1 wherein said means for extinguishing comprises means for applying a voltage to said electrodes whereby a d.c. voltage may be applied during operation to effect inhibition or switching off of a multipactor discharge set up between the electrodes.
 3. A magnetron oscillator according to claim 2 wherein said separate cavity is formed on the outer wall of the cathode and anode structure of the magnetron oscillator.
 4. A magnetron oscillator according to claim 3 wherein said separate cavity is coupled to one of the cavities in the cathode anode structure by a coupling loop.
 5. A magnetron oscillator according to claim 4 wherein one of said electrodes is provided with a heater.
 6. A magnetron oscillator selectively tunable to a first oscillating frequency and to a second oscillating frequency, said oscillator comprising in combination: magnetron anode-cathode resonator means for normally resonating at a frequency near to said first oscillating frequency, said resonator means including a magnetron resonant cavity; a separate resonant cavity capable of causing said resonator means to oscillate at said second oscillating frequency; coupling means coupling said magnetron resonant cavity to said separate resonant cavity for causing said oscillator preferentially to oscillate at said second selected frequency; multipactor discharge means in said separate resonant cavity for effectively short circuiting said separate resonant cavity whereby said oscillator oscillates at said first oscillating frequency; and means for selectively controlling said multipactor discharge means to select the frequency at which said oscillator osCillates.
 7. A magnetron oscillator as defined in claim 6 wherein said multipactor discharge means comprises spaced electrodes between which multipactor discharge tends to occur at said second oscillating frequency due to the magnitude of energy coupled to said separate resonant cavity by said coupling means.
 8. A magnetron oscillator as defined in claim 7 wherein said means for selectively controlling said multipactor discharge means comprises means for selectively applying a dc voltage to said electrodes to extinguish multipactor discharge therebetween. 